The invention relates to a method and a spectacle lens grinding machine for shape grinding the circumferential edge of spectacle lenses and optionally for subsequently grinding a facet with a spectacle lens grinding machine comprising a spectacle lens securing shaft for securing the spectacle lens and a grinding wheel that is controllably moveable relative to the lens securing shaft.
In order to perform the shape grinding of the circumferential edge of spectacle lenses and the subsequent facet grinding as fast as possible, the grinding pressure for a rpm of the grinding wheel selected as high as possible is adjusted to a value which allows grinding the desired shape of a spectacle lens starting from a circular lens blank into without damaging or even destroying the spectacle lens.
For a fixedly adjusted grinding pressure it may occur that the torque acting during grinding onto the spectacle lens blank is greater than the securing force for clamping the lens blank at the securing shaft so that the spectacle lens blank may be rotated relative to the securing shaft. Such a rotation, when a spherical spectacle lens without reading glass portion is ground, is of minimal impact when no displacement of the focal point of the lens blank from the axis of rotation of the securing shaft occurs; however, such a rotation should not occur at all when the lens blank has a precisely angularly arranged reading glass portion relative to the optical axis of the lens blank or is to be provided with a cylindrical or prismatic ground portion whose axis position relative to the arrangement of the spectacle lens within the spectacle frame is predetermined.
A rotation of the spectacle lens blank within the securing shaft can occur especially when spectacle lenses with a highly anti-reflective coating are machined because these lenses have an especially low frictional coefficient relative to the securing devices at the securing shaft or attached holding blocks or suction devices.
When the grinding pressure is decreased to such an extent that a rotation of the lens blank on the securing shaft can be prevented with certainty, the machining time of the lens blank will increase and the output of the spectacle lens grinding machine is thus reduced. A change of the grinding pressure as a function of the width of the edge of the spectacle lens such that the grinding pressure is increased when the width of the lens blank is greater and is lowered when the width of the lens blank is smaller, is known from European Patent Application 0 096 337. However, this type of controlling of the grinding pressure leads to the grinding pressure being greatest for myopic lenses when the radius of the spectacle lens is greatest and to grinding pressure becoming smaller and smaller with decreasing radius of the spectacle lens, while this known control for hyperopic lenses has the opposite effect. With this kind of control and the grinding pressure a slipping of the spectacle lens of the securing shaft due to the grinding pressure can only be avoided when for myopic lenses the grinding pressure at the greatest radius of the spectacle lens is adjusted to a value that prevents slipping which means that with a radius that decreases the grinding pressure is to be decreased also so that overall the machining time is increased. This disadvantage does not occur for grinding of hyperopic lenses; however, the grinding pressure controlled based on the width of the spectacle lens is not always optimized.
It is therefore an object of the present invention to provide a method and a spectacle lens grinding machine for shape grinding the circumferential edge of spectacle lenses and for an optional subsequent facet grinding step with which a slipping of the spectacle lens secured on the securing shaft can be reliably prevented and with which the grinding of the circumferential edge of the spectacle lens can be performed as quickly as possible without running the risk of breaking or damaging the spectacle lens.